The present invention relates to a recording disk data playback apparatus having a capability for playback of both analog and digital recorded data from recording disks, and in particular to a recording disk data playback apparatus equipped with improved switchings circuits for enabling connection of various combinations of audio signals, produced by demodulation of playback analog and digital signals, to two or more sets of output terminals.
Various types of recording disk data playback apparatus, sometimes referred to as disk players, have been developed. Until recently, such players were divided into two basic types. One type is utilized only for playback of video disks on which video and audio data are recorded in the form of analog signals. Such disks are sometimes referred to as LDs, and this abbreviation will be used for these in the following. With such disks, the video and analog signals are utilized to frequency-modulate a high-frequency carrier signal, and the resultant modulated signal is recorded on the disk. The other type of disk player is utilized for playback of digital audio disks, sometimes referred to as compact disks or CDs, and the latter abbreviation will be used hereinafter for such disks. CDs have audio data recorded thereon in the form of a digital signal of PCM (pulse-code modulation) type, i.e. a carrier signal is modulated by an encoded digital signal representing the audio data, and the modulated signal is recorded on the disk. However in recent years, a new type of recording disk (designated in the following as LDD) has been developed, as described in Japanese patent No. 58-45780) whereby an audio signal which has been digitized, e.g. by a method such as PCM, is converted into a pulse train of form suitable for disk recording, e.g. by applying the EFM (eight-to-fourteen) technique, and this pulse train signal is then superimposed upon a signal which has been produced by FM modulation of a high-frequency carrier by a video signal and an audio signal. The signal which results from this superimposition of the pulse train signal upon the modulated carrier is recorded on the disk. With the latter method, the audio signal is generally separated into two channels, e.g. corresponding to the stereophonic right and left channels, with 2.3 MHz and 2.8 MHz audio carriers being respectively frequency modulated by the two audio channel signals. The frequency spectrum of the recorded signal is such that the the sync tip portions of the video signal correspond to a frequency of 7.6 MHz, the pedestal level to 8.1 MHz, and the white peak level to 9.3 MHz. If the EFM techniqe is used to record the audio digital signal, then the frequency spectrum of the pulse train will extend from 3T to 11T, where T is the bit period of the PCM signal, 3T corresponds to a pulse frequency of approximately 720 KHz, and 11T is the maximum pulse width and corresponds to a frequency of approximately 200 KHz. This pulse train signal is superimposed on the main video carrier at a level which is approximately 1/10 off the carrier level, or less. Amplification and slicing close to the zero-crossing points are then performed to produce a pulse-width modulated signal, which is used as the recording signal.
With video and audio signals recorded on a disk by the method described above, the frequency spectrum of the RF (radio frequency) signal which is produced from the disk will be as shown in FIG. 1. Here, A denotes the digitized audio signal component, B denotes the audio FM signal component, C denotes the color information component of the video FM signal component, and D denotes the brightness component of the video FM signal component.
A very wide dynamic range, e.g. 90 dB or higher is provided by a digitized audio signal with such a system. Thus, a substantial improvement in acoustic fidelity can be attained, by comparison with recording and playback of audio signals using frequency modulation.
The present invention is directed towards a recording disk data playback apparatus which is capable of playback of disks having audio signals recorded thereon by digital modulation (CDs), disks having audio signals recorded thereon by frequency modulation (LDs), and disks of the type just described, having both digital and FM audio data recorded thereon (LDDs). When a LDD is played, then both audio signals derived from the digital demodulator system and audio signals derived from the analog (i.e. FM) demodulator system of the disk player will be produced, while when a CD disk is played, only audio signals from the digital demodulator system will be output, and when a LD is played then only audio signals from the analog (FM) demodulator system will be output. If two separate sets of output terminals are provided on the apparatus, for output of the analog and digital-derived signals respectively, with each set of output terminals comprising a L (left-channel) and R (right-channel) terminal, and if the user possesses only one set of loudspeakers, then the apparatus will be somewhat troublesome to utilize. That is to say, depending upon the type of disk to be played, the user would have to manually change over the output terminal connections to the loudspeakers. Thus, some type of output switching circuit arrangement which will offer maximum convenience and simplicity of operation is desirable for such a recording disk data playback apparatus.
In addition, it is desirable that means be provided for selecting various combinations of the digital-derived audio L and R channel signals and the analog-derived audio L and R channel signals to be applied to the audio output terminals of the apparatus and hence to the loudspeakers. For example, in some cases it is necessary to apply one audio channel (L or R) to both of the audio output terminals. This will be true, for example, when audio signals are being output which have been multiplexed for a purpose other than stereophonic sound reproduction, for example when one audio channel contains material in one language and the other channel contains the same material in a different language. In such a case, the user must have the capability for selectively applying either the L channel output to both sets of loudspeakers, or the R channel output to both sets. It is therefore necessary to provide suitable switching circuits to select the appropriate audio demodulator output signals to be transferred to the output terminals of the apparatus, e.g. to be supplied to loudspeakers. Due to the various combinations of types of disk which may be played, this switching system will be slightly complex, and in the case of playback of a LDD the audio output signal switching circuits will receive as input signals both the audio signals derived from FM demodulation and the audio signals derived from digital demodulation. Thus, although there is a capability for providing very high sound quality from the digitally modulated recorded audio signals, this sound quality may be lowered due to the effects of crosstalk within the necessary output signal switching circuits.